We have been on SaaS for over a year with our 6000 member community.
I cannot say a single bad thing about it.
We moved over from a heavily customized Telligent Community and they were able to handle the migration with ease. They have responded to our issues (Mainly ours, not theirs) very quickly, sometimes writing the customizations needed on the fly.
I normally get a response in less than 30 min, often within 15 min anytime of day.
They provide all updates, support, and migration assistance for the money.
If your on the fence… Allow me to push you over if I can!